GOLD
Gold (Au) is the most prestigious metal known (but not
the most valuable). Gold is the only metal that has a deep, rich,
metallic yellow color. Almost all other metals are silvery-colored.
Gold is very rare in crustal rocks - it averages about
4 ppb (parts per billion). Where gold has been concentrated, it occurs as
wires, dendritic crystals, twisted sheets, octahedral crystals, and
variably-shaped nuggets. It most commonly occurs in hydrothermal quartz
veins, disseminated in some contact- & hydrothermal-metamorphic rocks, and
in placer deposits. Placers are concentrations of heavy minerals in
stream gravels or in cracks on bedrock-floored streams. Gold has a high
specific gravity (about 19), so it easily accumulates in placer deposits.
Its high density allows prospectors to readily collect placer gold by panning.
In addition to its high density, gold has a high
melting point (over 1000º C). Gold is also relatively soft - about 2.5 to
3 on the Mohs Hardness Scale. The use of pure gold or high-percentage
gold in jewelry is not desirable as it easily gets scratched. The
addition of other metals to gold to increase the hardness also alters the
unique color of gold. Gold jewelry made & sold in America doesn’t
have the gorgeous rich color of high-purity gold.
Gold
(12 mm across)
Gold
(placer nugget) (9.5 mm across)
Gold placer
nugget from California, USA (public display, Carnegie Museum of Natural
History, Pittsburgh, Pennsylvania, USA).
Gold-quartz hydrothermal vein sample from California, USA (public display, Carnegie
Museum of Natural History, Pittsburgh, Pennsylvania, USA).
Gold from
quartz-gold hydrothermal veins in California, USA (public display, Carnegie
Museum of Natural History, Pittsburgh, Pennsylvania, USA).
Gold-quartz hydrothermal vein sample from a deep subsurface gold mine at
Witwatersrand, South Africa (public display, Carnegie Museum of Natural
History, Pittsburgh, Pennsylvania, USA).
Gold-quartz hydrothermal vein from the Trump Mine, Broad Arrow Mineral Field,
Western Australia.
Gold mass
from the Super Pit at Kalgoorlie, Western Australia.
Gold masses
from the Central City Mining District, Gilpin County, north-central Colorado,
USA (left: CSM 54583; right: CSM 54564, Colorado School of Mines
Geology Museum, Golden, Colorado, USA).
Gold
from the Eagles Nest Mine, Placer County, California, USA.
Gold
fluvial pebbles from Washingston State, USA (public display, Field Museum of
Natural History, Chicago, Illinois, USA).
Gold
- unusually-shaped gold mass from Canada (public display, Field Museum of
Natural History, Chicago, Illinois, USA).